Contemplate the \"z-scheme\" diagramed in Figure 10.15 on page 195 of your book.

Question

Contemplate the "z-scheme" diagramed in Figure 10.15 on page 195 of your book. The noncycllic electron flow generates ATP from the electrons excited from Photosystem II (PS II) and NADPH from the electrons excited from Photosystem l (PSI). Both of these compounds are needed to complete the dark reaction (a.k.a. the Calvin Cycle). Now look at the next page, 196. Figure 10.16 illustrates cyclic electron flow around PSI This produces ATP from the electrons excited by PSI. This suggests that both ATP and NADPH can be produced by PS l. So, what is the point of PS ll? In other words, why can't the chloroplast use cyclic electron flow around PSI to get the ATP it needs, and the noncyclic flow from PSI to get NADPH?

Explanation / Answer

Answer:

Photosynthesis is two stage process: one stage is dependent on the light and other is independent of it. The light reaction is a light dependent, in which the light energy is trapped by chlorophyll to make ATP and NADPH. The dark reaction is independent on the light in which the product of light reaction (ATP and NADPH) are used to make carbohydrates from CO2.

The light reaction involve process of photophosphorylation, which is of two types (in oxygenic photosynthetic organisms): Cyclic and non-cyclic.

The non-cyclic photophosphorylation, involve the formation of ATP coupled with a light-induced electron transport from H2O to ferrodoxin and a concomitant evolution of O2. It involves a collaboration of two photoreactions: a short-wavelength photoreaction (belongs to photosystem II) and a long-wavelength photoreaction (belongs to photosystem I). Electrons move from H2O through PS II to PS I and then to NADP+ (through Z scheme) . Thus, overall the non-cyclic photophosphorylation involve 3 processes:

All these process are tightly coupled and controlled. During the non-cyclic photophosphorylation process (involve both PSI and PSII in Z scheme), result in the production of 4 molecules of ATP per molecule of O2 evolved.

However, the cyclic photophosphorylation involves only PS I, yields only ATP and produce no net change in the oxidation-reduction state of any electron donar or acceptor.

In plants, the cyclic flow of electrons is utilized only when the reductive power (NADPH) that is needed to synthesize carbohydrates is high (and NADP+ is low), but still needs ATP to power other activities in the chloroplast. thus, there is no NADP+ reduction, but protons are pumped across the membrane and thus, ATP is generated.

Based on above information it is concluded that chloroplast needed both PSI and PS II to generate both NADPH and ATP in the sufficient amount (through tightly coupled and controlled redox reaction), to fullfill the need of plants to synthesize the food products (carbon products).

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